An examination of death penalty cases in Florida, one of the the three states with looser rules, found that a unanimous jury vote requirement would have substantially reduced the number of death sentences meted out.
If the state had required unanimity, between January 2010 and June 2015, there would have been 70% fewer death verdicts, in less than half as many counties.
But as a practical matter, too, unanimity should be a required condition for the death penalty. The fact is we just aren't very good at executions. Look at the current chaos among the states, notably Oklahoma, trying to "humanely" put down condemned prisoners in chemical executions lasting over half an hour. And then there are the terrifying results of the Innocence Project's efforts to free the wrongfully convicted from death row, In addition to taking a life, we may improperly impose a potentially excruciating death on an innocent person. This seems an unreasonably high cost to the goal of extinguishing a criminal's life.
By requring unanimous jury votes in capital cases we have the means to make the death penalty difficult for prosecutors to obtain and judges to impose. If in the process we execute fewer citizens, the guilty and innocent alike, we can at the least take grim comfort in knowing that we have limited the ability of the criminal justice system to impose its harshest penalty on those who least deserve it.